In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move into a position where the inductance of an energized phase winding is maximized. In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines are generally referred to as synchronous reluctance machines. In another type of reluctance machine, circuitry is provided for detecting the position of the movable part (generally referred to as a "rotor") and energizing the phase windings as a function of the rotor's position. These types of machines are generally known as switched reluctance machines. The present invention is applicable to both synchronous and switched reluctance machines.
The general theory of the design and operation of reluctance machines in general, and switched reluctance machines in particular, is known in the art and is discussed, for example, in Stephenson and Blake, "The Characteristics, Design and Applications of Switched Reluctance Motors and Drives", Presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
As explained above, the basic mechanism for torque production in a reluctance motor is the tendency of the rotor to move into a position to increase the inductance of the energized phase winding. In general, the magnitude of the torque produced by this mechanism corresponds to the magnitude of the current in the energized phase winding such that the motor torque is heavily dependent on the phase current waveforms. For an ideal motor with no magnetic saturation, the instantaneous torque T is: ##EQU1## Where i is the instantaneous current in the energized phase winding and dL/d.theta. is the derivative of the phase inductance L with respect to the rotor position 0. While all practical reluctance motors have some magnetic saturation this equation is useful for purposes of the present analysis.
For ideal torque production the phase energization currents would be substantially rectangular with each phase current terminating at the point the next phase current is initiated. In practice, rectangular phase currents are not obtainable and the ideal practical phase current waveforms are trapezoidal. The rate of change of the phase energization current is limited by the back emf generated by the rotating rotor since: ##EQU2## where V.sub.dc is the DC bus voltage of the power converter that provides the phase energization current, E.sub.mf is the back emf of the motor, and L is the inductance of the excited phase winding.
At high rotational speeds, the back emf produced by the rotating rotor E.sub.mf can significantly limit the ability of the power converter to provide trapezoidal energization currents. At these speeds the phase energization currents can become more triangular than trapezoidal.
These triangular phase energization currents can significantly diminish the performance of the motor. In particular, the triangular currents produce increased torque ripple and, as such, increased motor noise and vibration. Also, because of the decrease in the average phase excitation current, the power density, the performance and efficiency of the motor can be adversely affected.
In traditional reluctance machine systems the degradation of machine performance at high rotor speeds is often either ignored, avoided by avoiding high speed operation, or compensated for by increasing the DC bus voltage of the power converter. None of these "solutions" to the performance degradation are ideal. Ignorance and avoidance sidesteps the problem, and increasing the power rating of the converter both increases the overall costs of the system (by requiring higher voltage devices) and tends to decrease the overall efficiency of the system.
One suggested approach to improving the phase current waveforms was presented in U.S. Pat. No. 5,459,385. In this approach, a full-pitch commutation winding is placed in the stator of a reluctance motor and the commutation winding is coupled to the phase windings such that, upon commutation of an energized phase winding, the commutation winding absorbs, through mutual inductance, some or all of the energy stored in the magnetic field established by the energized winding. This transference of energy is purported to force the current in the commutated phase winding to drop to zero faster, thus allowing the commutation of the phase winding to occur closer to the ideal point of rotor pole/stator pole alignment. While this approach may have some advantages, it results in an commutation winding that is energized by discontinuous currents, where the current pulses applied to the commutation winding occur at the commutation points of the phase windings. These current discontinuities can result in significant flux changes in the motor which can negatively impact the motor's performance. Moreover, in these machines, the commutation winding acts as an essentially "parasitic" device in that it absorbs and, potentially stores, energy from the motor. It does not provide any significant useful energy to the motor.
It is an object of the present invention to overcome the referenced limitations, and other limitations, in the prior art by providing an increased performance reluctance machine system that has improved phase energization current waveforms; reduced noise and vibration; improved power density; and improved performance in terms of efficiency over traditional reluctance machines. It is a further object of the present invention to accomplish these improvements without significantly increasing the power rating of the converter used in the system.